How Heat Transfer Creates Weather Systems Professor Jeff Gawrych De Anza College

Since the Tropics are warm and the Polar Regions are cold, air will start to circulate.

We know...  Earth is warmed via... – Radiation: amount of sunlight absorbed – Conduction: significant, but minor contribtuion – Convection: Air movement (wind)/Weather systems –Greenhouse Effect: Traps outgoing radiation  Air is a poor conductor of heat  Earth cools primarily by emitting longwave, IR radiation  Therefore, daily temperatures are determined by how strong or weak these processes are on any given day. –We can assume that the rates of radiation and conduction will not change from day-to-day.

Daily temperatures  Day-to-day variation in temepratures is mainly to do with convection and the G.H. effect  Exercise: Make a list of some things that you think are necessary to accurately forecast tomorrow’s high and low temperatures in San Jose.  Maybe it will help to think along these lines: –Why will the high temperature probably NOT be 110 degrees tomorrow?  Why will the low temperature probably NOT be 80 degrees tomorrow?

Daily temperatures  A few answers: – The most imporant factor in determing on how hot or cold it can get in a given day is the profile of the air mass above us.  We get this info from weather balloons  So knowing the current conditions is essential! –Cloud cover: During the day, clouds block incoming sunlight. At night, they enhance the greenhouse effect –Winds: winds limit how much a place can warm up or cool off –Land use: Urban vs. Rural locations: Urban areas get warmer during the day/stay warmer at night

Air Masses  A general description of the atmosphere over a certain area. –Tells temperature and moisture profile.  Type of air mass over a region tells what the weather may be like.  Air masses originate in specific areas and can dominate the climate by not moving for long periods of time . –Remember: climate is what you expect, weather is what you get.

Air masses  Name of air masses tells you where it originated  c = continental = dry  m = marine = moist  T = tropical = warm  P = polar = cold  A = Arctic = extremely cold  Examples cT: continental tropical (warm and moist) cP: continental polar (cold and dry) mT: marine tropical ( warm and moist) mP: marine polar (cold and moist)

Pressure Differences  Determines the direction and speed of winds  Can help explain circulation patterns around the world  General guidelines: –High pressure: clear skies/fine weather –Low pressure: cloudy skies/changeable weather  We will see why this is later...

It’s the difference in pressure that causes a fluid like air to move.

What causes pressure to change?  Temperature changes!!  Therefore Density change Formally, the equation of state describes this relationship: p=  RT p-Pressure (Pa)  - density (g m -3 ) R - Gas Constant (287 J K -1 kg -1 ) T - Temperature (K) Cold air is ____ dense and warm air is ____ dense, so cold air _____ and warm air _____!!!

Fronts  Separate different air masses of different densities. Cold air is more dense than warm air  cold air sinks, warm air rises.  So when a cold front passes, it forces lifting. –Cold fronts bring cold air into region –Warm fronts bring warm air into region

Cold Front Warm front